The study and treatment of respiratory sleep disorders, such as sleep apnea, has resulted in the development of numerous diagnostic, monitoring and treatment devices. One type of these devices is the Positive Air Pressure (“PAP”) device, in which the patient receives a controlled amount of gas depending on a variety of factors. For example, in a Bi-Positive Air Pressure (“Bi-PAP”) device, the patient receives gas at one pressure during inhaling, and at another pressure while exhaling. PAP devices are primarily used for treatment of respiratory sleep disorders, but can be used for diagnostic and monitoring functions as well.
Devices such as the Bi-PAP device must, therefore, have the ability to quickly determine the patient's respiratory state and apply the correct gas pressure associated therewith.
Previous attempts to provide this ability in positive air pressure devices have involved the use of “on-off” switches or valves as flow sensors. Other methods include the use of pressure transducers, which can be cumbersome and are generally not cost effective.
One problem associated with the switches or valves is that they cannot provide an analog signal indicative of the patient's breathing force. Another problem with existing flow sensors is that at low fluid flow rates or pressures, the sensor sensitivity is low, and the sensitivity only improves as the flow increases. For the treatment of sleep disorders such as sleep apnea, it is especially important for these devices to be able to detect and respond to very low fluid flow rates.
There is a need for a flow sensing device which can generate an analog signal in response to a patient's breathing, has a high sensitivity at a low fluid flow pressure, and is economical and compact so as to be suitable for use with a variety of positive air pressure devices.